The present invention relates to catalysts useful to form portions of electrolysis anodes. More particularly, it relates to cobalt/oxygen/buffering electrolyte (e.g. cobalt/oxygen/fluorine) based catalysts suitable to facilitate water electrolysis.
The search for compositions to catalyze electrolysis of water is primarily currently driven by the desire to store renewable energy (e.g. solar or wind energy) in the form of hydrogen gas, with the hydrogen gas then becoming a more practical substitute for fossil fuels in applications such as automobiles. FIG. 1 depicts schematically a prior art water electrolysis system. A container 2 stores an aqueous solution 3. An anode 4 and a cathode 6 are positioned in a water based electrolyte solution and then linked to a current source (not shown). A diaphragm isolates the gases developed by splitting water into its constituent gases.
This prior art technology generates oxygen and hydrogen in this application. However, it does so in such an energy inefficient manner so as to render the process commercially impractical as a means of converting solar energy or the like to hydrogen gas fuel. In this regard, the electrolytic gas production involves transfer of four protons and four electrons with the formation of an oxygen-oxygen bond at the anode concomitant with reduction of protons at the cathode. A substantial amount of energy to drive that reaction at some useful rate must be provided over the theoretical minimums required (the “overpotential”).
Efforts have been made to try to reduce the amount of overpotential needed to drive the reaction by using specialized anodes and/or a catalyst. This has helped somewhat. However, there are still significant commercial impediments to implementing their use.
For example, some catalysts degrade under the reaction conditions required. Others are not widely available at reasonable cost, or do not reduce the required overpotential sufficiently.
Some cobalt oxide materials have been tried as water-electrolysis catalysts in hydroxide electrolyte systems. These consist of CoIII oxide clusters which are active in strongly basic media. They appear to proceed via a process involving CoII, CoIII, and CoIV-oxo species. These require basic conditions to efficiently function, as hydroxide is both the electrolyte and buffer, and must operate at elevated temperatures for optimal efficiency. Cathode driven reactions (e.g. the formation of hydrogen from water, or the conversion of carbon dioxide gas to methanol) have specific pHs for their most efficient production. These do not correlate with the conditions of this prior art cobalt system.
In U.S. Pat. No. 3,399,966 there was a disclosure of a cobalt oxide coating on an electrolysis anode, used in one example with fluoborate electrolyte. However, this did not adequately address the overpotential concern.
In unrelated work it is known that CoF3 and fluorocobaltateIII salts react with water to liberate oxygen and HF. See H. Priest, Anhydrous Metal Fluorides, 3 Inorg. Syn. 171-183 (1950); V. Ustinov et al., Separation Of Oxygen Isotopes In The Fluorination Of Oxygen-containing Compounds, 52 Zh. Fiz. Khim. 344-347 (1978); V. Klemm et al., Über Fluorocobaltate(III) und -(IV) und Fluoroniccolate(III), 308 Anorg. Allg. Chem. 179-189 (1961).
Further, there have been some attempts to describe aqueous and non-aqueous solutions containing both cobalt and fluoride ions in the context of electrochemical studies. See A. Kappanna et al., Anodic Reactions In The Electrolysis Of Acid-Cobalt-Fluoride, 18 Current Science 27 (1958); B. Cox et al., Complex Fluorides . . . , J. Chem. Soc. 1798-1803 (1954); M. Meyers et al. The Preparation, Structures . . . , 82 J. Am. Chem. Soc. 5027-5030 (1960); and T. Court et al., Solution Chemistry Of Cobalt In Liquid Hydrogen Fluoride, 6 J. Fluorine Chem. 491-498 (1975).
The production of a water oxidation catalyst by electrolysis of solutions of Co2+ salts in aqueous phosphate, borate, and methylphosphonate buffers has also recently been reported. M. Kanan et al., In Situ Formation Of An Oxygen-Evolving Catalyst In Neutral Water Containing Phosphate And Co2+, 321 Science 1072-1075 (2008); and Y. Surendranath et al., Electrolyte-Dependent Electrosynthesis., 131 J. Am. Chem. Soc. 2615-2620 (2009). However, the required overpotential at useful current densities is a significant impediment to commercial application and the pH of the system is limited to neutral or mildly alkaline values.
Hence, there still is a need for improvements for converting water to oxygen and hydrogen in electrolysis reactions